[2.01] Recent Radar Astrometry of Asteroid 2004 MN4

Arecibo (2380-MHz) delay-Doppler radar astrometry obtained
in late January of 2005 significantly corrected 2004 MN4's
orbit. Doppler-shifted echoes were acquired 4.8-sigma away
from the predicted frequency on Jan 27, while range to the
object on Jan 29 was found to be 747 km (2.8-sigma) closer
to Earth than the pre-radar orbit solution predicted.
Incorporation of these radar measurements into least-squares
orbit solution #82 resulted in a new predicted Earth
encounter on 2029-Apr-13 of 36000 +/- 9900 km (3-sigma
formal uncertainties), or 5.6 +/- 1.6 Earth radii, from
Earth's center. This is inside geosynchronous orbit and
27700 km (4.3 Earth radii) closer to Earth than predicted by
the pre-radar ephemeris -- a 5-sigma change compared to the
pre-radar orbit solution, illustrating the problematic
nature of prediction and statistical analysis when only
single-apparition optical data-sets are available. The
current data-set does not permit reliable trajectory
propagation to encounters later than 2029; this may not be
possible until data from 2012-2013 are available. The
corrected nominal approach distance in 2029 is approximately
twice the classical Roche limit and closer than any known
past or future approach by a natural object larger than ~10
m, other than those detected after already impacting the
Earth or it's atmosphere. Such close approaches by objects
as large as 2004 MN4 (D \gtrsim0.3 km) are currently
thought to occur at \gtrsim1000-year intervals on average.
2004 MN4 is expected to reach 3rd magnitude for observers in
Europe, western Asia, and Africa, and thus be visible to the
unaided eye. The asteroid's disk will be 2-4 arcseconds
across and potentially resolvable with small ground-based
telescopes.